Generally, silver halide emulsions used in silver halide photographic materials are chemically sensitized by various chemical substances to obtain desired sensitivity, gradation, etc. Typical examples of conventional sensitization methods include sulfur sensitization, selenium sensitization, noble metal sensitization such as gold sensitization, reduction sensitization and combinations thereof.
Recently, there has been a demand for silver halide photographic materials having high sensitivity, excellent graininess and sharpness. Further, there has been a demand for materials which may be rapidly processed, for example, in an expedited development. Hence, various improvements in the above sensitization methods have been made.
Among the above-described sensitization methods, is the selenium sensitization method disclosed in U.S. Pat. Nos. 1,574,944, 1,602,592, 1,623,499, 3,297,446, 3,297,447, 3,320,069, 3,408,196, 3,408,197, 3,442,653, 3,420,670 and 3,591,385, French Patents 2,093,038 and 2,093,209, JP-B-52-34491 (the term "JP-B" as used herein means an "examined Japanese patent publication"), JP-B-52-34492, JP-B-53-295, JP-B-57-22090, JP-A-59-180536 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-59-185330, JP-A-59-181337, JP-A-59-187338, JP-A-59-192241, JP-A-60-150046, JP-A-60-151637, JP-A-61-246738, U.K. Patents 255,846 and 861,984 and H. E. Spencer, et al., Journal of Photographic Science, Vol. 31, pages 158 to 169 (1983).
Generally, selenium sensitization causes great fogging, though selenium sensitization exhibits sensitization efficiency higher than that of sulfur sensitization methods conventionally carried out in the art. Further, selenium sensitization is apt to result in low contrast. Most of the above-described patents are directed to improve these disadvantages. However, sufficient results have not yet been obtained, and there has been a great demand for fundamental improvements in the prevention particularly of fogging.
Further, when sulfur sensitization or selenium sensitization is carried out in combination with gold sensitization, a remarkable increase in sensitivity can be obtained and at the same time, the degree of fogging is increased. Gold-selenium sensitization causes a remarkable rise in fog particularly in comparison to gold-sulfur sensitization. Accordingly, there has been a strong demand for a method for preventing fogging and for a selenium sensitizing agent which scarcely causes fogging.
The above-described patents fail to disclose a method for preventing fog from being caused by selenium compounds described in those patents.